Exciton collective modes in a bilayer of axion insulator $\text{MnBi}_2 \text{Te}_4$

TL;DR

This paper explores the formation and properties of exciton condensates and their collective modes in a bilayer of the topological magnetic insulator MnBi2Te4, highlighting the effects of external tuning and thermal fluctuations.

Contribution

It introduces a theoretical framework for exciton condensation in MnBi2Te4 bilayers, analyzing collective modes and spectral functions with potential experimental relevance.

Findings

Identification of conditions for exciton condensation.

Prediction of collective mode softening with detuning and temperature.

Derivation of exciton spectral function for experimental comparison.

Abstract

We investigate the emergence of an exciton condensate and associated collective modes in a bilayer configuration of $\text{MnBi}_2\text{Te}_4$, an antiferromagnetic topological insulator and van der Waals material, recognized for hosting axion physics. Utilizing a minimal low-energy Hamiltonian for the two layer system which is gapped by the intrinsic N\'eel order, we first employ mean-field theory to establish the conditions for exciton condensation. Our analysis identifies a nonzero, spin-singlet exciton order parameter which is tuned by external displacement field, temperature, and Coulomb attraction. Beyond the mean-field, we explore collective mode fluctuations in the uncondensed phase via many-body perturbation theory and the random phase approximation. From this, we derive the exciton spectral function which allows for a direct comparison between theoretical prediction and experimental observation. We detail how the softening of the collective mode peak is a function of the competition between interlayer detuning and thermal fluctuations. This work elucidates how the unique topological and magnetic environment of $\text{MnBi}_2\text{Te}_4$ offers a tunable platform for the realization and manipulation of exciton condensates and the corresponding collective excitations. Our findings contribute to understanding the interplay of topology and bosonic condensates, which could inspire application in optically accessing topological properties, dissipationless transport, and gate-tunable optoelectronics.